CVE-2025-39682: Linux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability

Vulnerability summary

CVE-2025-39682 is a critical (CVSS 9.8) vulnerability in Linux Kernel. It is known to be exploited: CISA lists it, first on 18-09-2026. EPSS estimates a 3.25% chance of exploitation in the next 30 days, higher than 88.3% of scored vulnerabilities.

Timeline

1. 05-09-2025
   PublishedVulnerability first disclosed.
2. 18-09-2026
   Added to CISA KEVExploitedLinux Kernel Improper Check for Unusual or Exceptional Conditions Vulnerability
3. 21-09-2026
   CISA remediation dueDeadline passedApply mitigations per vendor instructions.
4. 07-10-2026
   Last modifiedVulnerability information updated.

Description

In the Linux kernel, the following vulnerability has been resolved: tls: fix handling of zero-length records on the rx_list Each recvmsg() call must process either - only contiguous DATA records (any number of them) - one non-DATA record If the next record has different type than what has already been processed we break out of the main processing loop. If the record has already been decrypted (which may be the case for TLS 1.3 where we don't know type until decryption) we queue the pending record to the rx_list. Next recvmsg() will pick it up from there. Queuing the skb to rx_list after zero-copy decrypt is not possible, since in that case we decrypted directly to the user space buffer, and we don't have an skb to queue (darg.skb points to the ciphertext skb for access to metadata like length). Only data records are allowed zero-copy, and we break the processing loop after each non-data record. So we should never zero-copy and then find out that the record type has changed. The corner case we missed is when the initial record comes from rx_list, and it's zero length.

• CWE-754Improper Check for Unusual or Exceptional ConditionsThe product does not check or incorrectly checks for unusual or exceptional conditions that are not expected to occur frequently during day to day operation of the product.